1. Field of the Invention
This invention relates to a method and apparatus for generating electric power in a downhole environment, and in particular to a method and apparatus for generating electric power using a thermoelectric device by converting a downhole temperature gradient into electric power.
2. Description of the Related
Battery packs are used downhole in wells for providing power to downhole electrical systems, system electronics, and other downhole devices or tools. The electrical loads connected to the battery packs require relatively high discharge rates that result in short battery pack life spans. Moreover, the hostility (heat, pressure, and the like) of the downhole environment also contributes to a shorter life span.
Generally, the downhole electrical systems powered by these battery packs are used during early stages of the well completion and are primarily to test aspects of the well. The life span of the battery packs are critical. In many installations in hostile downhole environments, a battery pack typically has a fourteen-day life span. If a well completion job or test job is delayed or the battery pack life span is too short for the job, the well tool must be retrieved from downhole, the battery pack is replaced and the well tool is replaced downhole to continue the completion or test job. This process is expensive (i.e. well intervention, additional labor and battery packs) and time-consuming.
Downhole electrical systems are becoming increasingly complex and requiring larger electrical loads. These larger and more powerful systems decrease even further the typical life span of a battery pack due to the increased energy consumption. The complexity and usefulness of downhole electrical systems is rapidly expanding to include downhole sensors, surface reconfigurable flow controls, data telemetry systems, and other electrical monitoring and sensing systems.
Downhole electronic sensors--such as transducers--are used to gather data useful in determining the operation of controls needed to manage the production processes in a well. Electronic sensors detect conventional and specialized subterranean formations and downhole parameters such as pressure data, temperature data, and near-bore flow effect measurements. This data is used to determine how a particular subterranean formation should be managed to maximize well production. Such sensors may have significant electrical power requirements depending on the nature and volume of the data, and sometimes require electrical energy at greater levels for extended periods of time.
Surface reconfigurable flow controls remain downhole after well completion for managing the production flow of subterranean formations, preferably for the production life of the well, which can potentially span decades. Conventional battery packs are unable to provide such long-term energy requirements.
Telemetry systems provide communications between downhole sensors or mechanisms and the well-site surface. At present, wireless telemetry systems have been used for only short term applications such as for initial production well tests. Battery life determines the length these systems may remain operable, and conventional battery packs have been unable to provide the necessary electric power for wireless signal transmission from a downhole environment for extended periods because wireless transmissions from downhole to the surface require a large amount of energy--on average 500 to 800 watts--because of the need to transmit and to propagate the signals through the earth. In comparison, wireless receptions downhole of a signal transmitted from the surface through the earth require only about 100 milliwatts of electrical energy. This is still a significant level of power when it is needed on a continuous basis.
The only alternative to battery packs is to use conventional power cables. However, cables are highly susceptible to failure in hostile downhole environments. For example, cables are generally carried on the exterior of a tubing string to the downhole sensor-deployment site. In most cases, the deployment sites are miles below the earth's surface. During the journey, severe friction and shear forces are present that can break or disable the cable. Also, if the downhole electrical system (such as a formation test or control apparatus) must travel a long distance, then the cable may break under its own weight. Furthermore, the internal electrical resistance (per unit-length) attenuates the power signal before it reaches the downhole electrical systems, thus requiring increased power at the surface. Additionally, the ambient fluids, typically at elevated pressure and temperature, may invade the shielding causing premature failure.
Thus, there exists a need for a self-sustaining, downhole electric power source that can provide power for downhole test, well completion, and flow control devices. Also, a need exists for a downhole electric energy source to recharge downhole battery packs and thus extend the life span of the battery packs.